Oxygen is produced industrially in enormous quantities from air. Currently, a majority of industrially-produced oxygen is separated from air by condensing the air and then fractionally distilling the liquid air to separate the oxygen from nitrogen and other gases. This liquefaction procedure consumes very large amounts of energy, since the boiling point of oxygen at atmospheric pressure is only 77.degree. K.
In view of the known disadvantages of the air liquefaction process, attention has recently been directed toward methods for the separation of oxygen from air at temperatures much closer to ambient. In principle, such separation methods are very simple; a solution is prepared containing a compound which can complex molecular oxygen in a manner similar to that of the known biological oxygen-complexing proteins, myoglobin and hemoglobin, this solution is exposed to air or a similar oxygen-containing gas such that a large proportion of the oxygen-complexing compounds become complexed with oxygen. The solution is then removed from contact with the air and exposed to an environment induced by pressure or temperature changes in which the oxygen partial pressure is less than that in equilibrium with the oxygen-complexed compound, so that the compound gives up at least part of its oxgyen, thereby releasing into the environment a gas much richer in oxygen than that with which the solution was originally in contact.
One technique for separating oxygen from air involves the use of "immobilized liquid membranes". Such immobilized liquid membranes comprise a solid support, typically a synthetic polymer which is inert to oxygen, together with liquid immobilized within the inert support. The support may have very fine pores therein so that the liquid is contained therein by capillary forces, a polymer film acting as the support may be swollen by contact with the liquid to form a gel or various other techniques may be used for immobilizing the liquid within the support. Air or some other oxygen-containing gas is passed over one side of the immobilized liquid membrane, while the gas which passes through the membrane is removed by pumping on the opposite side of the membrane. The oxygen "diffuses selectively" through the liquid membrane, due to the presence of an oxygen partial pressure gradient between the two sides of the membrane. The oxygen molecules are carried in the form of a metal complex through the immobilized liquid membrane at a much greater net transport rate than the rate in which other gases are passed through the membrane. One such membrane system is disclosed in U.S. Pat. No. 3,396,510 which discloses a facilitated transport system using a liquid membrane and a non-volatile species which is soluble in the immobilized liquid which reversibly reacts with a specific gaseous component to be separated from the gaseous mixture. Although the patent discloses the possibility of facilitated transport of oxygen, the proposed system is primarily an aqueous-based one, utilizing water soluble complexing agents, and was found to be commercially unfeasible.
Daryle H. Busch, et al. in an article entitled "Molecular Species Containing Persistent Voids. Template Synthesis and Characterization of a Series of Lacunar-Nickel Complexes in the Corresponding Free Ligands", in J. Am. Chem. Soc. 103 pp 1472-1478 (1981), discloses a family of lacunar ligands synthesized in the form of nickel (II) complexes by a template process. The species disclosed were designed to provide a "lacuna" or protective void, or cavity, in the vicinity of a coordination site in order to facilitate the binding of small molecules to the metal ions. The species of complexes are characterized by having four N-atoms bound to a single nickel atom in a ligand system which results in an overall +2 charge for the complex.
Kuninobu Kasuga, et al. in an article entitled "A Preparation and Some Properties of Cobalt (II) Schiff-base Complexes and Their Molecular Oxygen Adducts", Bull. Chem. Soc. Jpn. 56, pp 95-98 (1983) disclose seven new cobalt (II) complexes with a tetradentate Schiff-base ligand and their three oxygen adducts. The disclosed complexes are reported to be stable at room temperature for several weeks and have the characteristic of having favorable affinity for molecular oxygen.
Roman, in U.S. Pat. Nos. 4,451,270 and 4,542,010 disclose processes and an apparatus for the separation and purification of oxygen and nitrogen. The processes utilize novel facilitated transport membranes to selectively transport oxygen from one gaseous stream to another, thereby leaving nitrogen as a by-product. In accordance with this process, an oxygen carrier capable of reversibly binding molecular oxygen is dissolved in a polar organic solvent and the resulting carrier solution is contained within a membrane which separates a gaseous feed stream, such as atmospheric air, to form a gaseous product stream. The oxygen carriers employed in the disclosed process are metal-containing complexes wherein a metal is bound by four ligating atoms, and has the capacity to reversibly bind oxygen and is also soluble in various polar organic solvents and reactive with axial bases.
U.S. Pat. No. 4,584,359 discloses a membrane of a vinyl polymer which contains oxygen-transferring groups not in solution, but in a chemically bonded form, which is used for separating molecular oxygen from a mixture of gases.